1. Field of the Invention
The present invention relates to a transmission line, and more particularly, to a channel structure including a serpentine guard trace for reducing receiving-end crosstalk caused by electromagnetic interference of a signal of a nearby transmission line when transmitting a high speed signal through a transmission line on a printed circuit board.
2. Description of the Related Art
Far-end crosstalk refers to an interference between a signal transmitted through one transmission line and a signal transmitted through a nearby transmission line when transmitting the signal using a plurality of pairs of transmission lines. The far-end crosstalk is a phenomenon due to an electromagnetic interference and causes serious signal loss when transmitting a high speed signal.
FIG. 1 is a schematic diagram illustrating the far-end crosstalk occurring in the other transmission line 120 when a signal is applied to one of two nearby transmission lines 110 and 120. The transmission line to which the signal Va is applied is called an aggressor line 110, and the transmission line to which no signal is applied is called a victim line 120. The far-end crosstalk is caused by inductive coupling due to a mutual inductance Lm and capacitive coupling due to a mutual capacitance Cm shown in an electrical element model of FIG. 2. The far-end crosstalk is divided into crosstalk propagating to a transmitter and crosstalk propagating to a receiver. When the crosstalk reaches both ends, the crosstalk becomes receiving-end crosstalk (VFe) and transmitting-end crosstalk (Vne), respectively. In FIG. 1, Va is a voltage applied to the transmitting end of the aggressor line, and TD is a transmission time for the transmission. In FIG. 2, CS is a self capacitance per unit length, and LS is a self inductance per unit length. In FIG. 1, when it is assumed that a termination resistor R0=Z, Mathematical Expression 1 below is derived.
                              Vne          ⁡                      (            t            )                          =                              1            4                    ⁢                      (                                          Cm                Ct                            +                              Lm                Ls                                      )                    ⁢                      (                                                            V                  a                                ⁡                                  (                  t                  )                                            -                                                V                  a                                ⁡                                  (                                      t                    -                                          2                      ⁢                      TD                                                        )                                                      )                                              [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          1                ]                                          Vfe          ⁡                      (            t            )                          =                              1            2                    ⁢                      (                                          Cm                Ct                            -                              Lm                Ls                                      )                    ⁢                                    δ              ⁢                                                          ⁢                                                V                  a                                ⁡                                  (                                      t                    -                    TD                                    )                                                                    δ              ⁢                                                          ⁢              t                                                          [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          2                ]            
Here, l is a length of the transmission line, TD is a transmission time for the transmission line, Cm is a mutual capacitance per unit length, Ct is a sum of self capacitance and mutual capacitance per unit length, Lm is a mutual inductance per unit length, and LS is a self inductance per unit length. In addition, Va(t) is a voltage applied to the transmitting end of the aggressor line, Va(t−TD) is a voltage applied to the transmitting end of the aggressor line after TD, and δVa(t−TD)/δt is a variation of Va(t−TD) according to time t. In addition, R0 is a termination resistor disposed at both ends of the two signal lines, and Z is a resistance value which is the same as a resistive component of a characteristic impedance of the signal line.
The transmitting-end crosstalk (Vne) occurs continuously with a constant level while the signal propagates to the receiving-end as shown by Mathematical Expression 1. Accordingly, the transmitting-end crosstalk can be easily removed by digital calibration and so forth. However, the receiving-end crosstalk (VFe) instantly occurs only when an applied signal is changed, as shown by Mathematical Expression 2. Accordingly, it is difficult to remove the receiving-end crosstalk in a circuit. On the other hand, in case of a transmission line in a homogeneous medium such as a strip line, capacitive coupling strength is equal to inductive coupling strength. Ideally, Mathematical Expression 2 is equal to 0. Accordingly, in a system where the receiving-end crosstalk is a serious problem, the strip line is used as the transmission line to solve the problem. However, the strip line is more expensive in production than the micro-strip line. Therefore, the micro-strip line is used for a general digital system.
In case of the micro-strip line of which one side is exposed to air, the inductive coupling is stronger than the capacitive coupling. The coupling exponentially decays as the distance between the nearby transmission lines becomes larger. Therefore, it is possible to reduce the receiving-end crosstalk by sufficiently separating the nearby transmission lines one from another. To verify this, in a cross sectional view of a printed circuit board of FIG. 3, the receiving-end crosstalk is measured by alternating the distance S of the nearby transmission lines between 14 mil and 42 mil, to obtain the result of FIG. 4. In addition, as shown in FIG. 3, a dielectric material such as FR-4 (Flame Retardant-4) having a dielectric constant ∈ of 4.5 may be used for the printed circuit board. In FIG. 3 ‘mil’ is a unit of length, 1 mil is equal to 1/1000 inch, and ‘Air’ means that one side of the transmission line is exposed to air. ‘1.4 mil’ is a thickness of a printed circuit board, ‘8 mil’ is a thickness of a dielectric material, ‘0.7 mil’ is a thickness of the transmission line, ‘14 mil’s are width of the transmission lines, and ‘S’ is the distance between the nearby transmission lines. In FIG. 4, the vertical axis represents the strength of crosstalk of the receiving end in units of mv, and the horizontal axis represents the transmission time in units of ps.
Referring to FIG. 4, as the distance between the nearby transmission lines is increased, the receiving-end crosstalk is reduced, and, however, the considerable receiving-end crosstalk remains. In FIG. 4, a solid line represents the far-end crosstalk in case of the distance between the nearby transmission lines is 14 mil, and a dotted line represents the far-end crosstalk in case of the distance between the nearby transmission lines is 42 mil. To reduce the remaining receiving-end crosstalk, as shown in FIG. 5, a guard trace is located between the nearby transmission lines as in the conventional methods. FIG. 6 shows a cross sectional view of a printed circuit board for verifying the reduction of the receiving-end crosstalk of the guard trace structure of FIG. 5. In FIG. 5, characteristic impedance of the guard trace between the nearby transmission lines is generally equal to that of the signal line. Accordingly, a termination resistor of the guard trace has the same resistance with other signal lines. In FIG. 5, TD is a transmission time for the transmission line, Z is a resistance value which is the same as a resistive component of a characteristic impedance, Va1 is a voltage applied to the transmitting end of the aggressor line, R0 is a termination resistance of the transmission line, R1 is a termination resistance of the guard trace, and VFe is a crosstalk of the receiving end of the victim line. In addition, as shown in FIG. 6, a dielectric material such as FR-4 (Flame Retardant-4) having a dielectric constant ∈ of 4.5 may be used for the printed circuit board. In FIG. 6 ‘Air’ means that one side of the transmission line is exposed to air, ‘1.4 mil’ is a thickness of a printed circuit board, ‘8 mil’ is a thickness of a dielectric material, ‘0.7 mil’ is a thickness of the transmission line. ‘14 mil’s represent the width of the transmission lines, the width of the guard trace, and the distance between transmission line and guard trace.
FIG. 7 shows a measurement result of the receiving-end crosstalk with respect to time axis to verify an effect of the guard trace of FIGS. 5 and 6. In FIG. 7, the vertical axis represents the strength of crosstalk of the receiving end in units of mv, and the horizontal axis represents the transmission time in units of ps. In FIG. 7, a solid line represents the receiving-end crosstalk without guard trace, and a dotted line represents the receiving-end crosstalk with conventional guard trace. As shown in FIG. 7, when the conventional guard trace is used, the receiving-end crosstalk is partially reduced, however, the system performance cannot be remarkably improved.